Turbines are used for generating rotary mechanical power from the energy in a working fluid. The working fluid energy, originally in the form of pressure energy, is converted to velocity energy by passing through a system of blades in the turbine. Changes in the magnitude and direction of the velocity energy are made to cause tangential forces on the blades, producing mechanical rotation of a turbine rotor. The rotating turbine rotor may be positioned to interact with a generator rotor and generator stator and thereby produce electricity.
FIG. 1 is a simplified illustration of a rotor assembly 20 that includes a rotor shaft 22 supporting a number of blade assemblies 24A, 24B, 24C, 24D. Each blade assembly 24 includes a blade hub 26A that supports a number of blades 28.
A rotor blade attachment structure, to be described below, couples the blade hub. 26A with the blades 28. The attachment structure requires periodic inspection to detect service induced flaws. As used herein, service induced flaws refer to any type of flaw that may initiate during the operation of a turbine rotor, including stress corrosion cracking, creep fatigue cracking, fatigue cracking, pits, and other imperfections generally arising from stress and exposure to corrosive environments.
If the blades 28 are removed from the hub 26A, the geometry of the attachment structure is exposed. Consequently, service induced flaws may be identified by liquid penetrant, magnetic particle, and eddy current inspection techniques. These techniques may also be used when the blades do not entirely cover the attachment region structure.
A widely used rotor blade attachment structure is the straddle-mount design. In the straddle-mount design, the blade straddles the entire attachment structure. As a result, surface inspections can only be conducted with the blades removed, a time-consuming and expensive task.
FIG. 2 shows a straddle-mount rotor blade attachment architecture 29. The straddle-mount rotor blade attachment architecture 29 includes a slot region 30, with a number of slots 31A, 31B, and a straddle-mount region 32 including a number of hooks 33A, 33B. FIG. 2 also depicts individual blades 28A, 28B. Each blade 28 includes a blade interlock structure (34A or 34B) and a blade face (36A or 36B).
It can be appreciated from FIG. 2 that a blade interlock structure (34A or 34B) is fitted over the slot region 30 and is then moved to the straddle-mount region 32 where it forms a secure fit with the straddle-mount region 32 hooks 33A, 33B. In this way, blades 28 are positioned around the entire periphery of the blade hub 26. The last blade 28 placed on the hub 26 is positioned at the slot region 30 and is secured at the slot region by pinning it to the blade hub or attaching it to adjacent blades.
FIG. 3 is an enlarged perspective view of a straddle-mount rotor blade attachment structure. The figure more particularly illustrates the nature of the straddle-mount region 32 and its corresponding hooks 33A, 33B. The figure also illustrates the slot region 30 and its slots 31A, 31B. As used herein, the straddle-mount region 32 and the slot region 30 include the shaped perimeter of the blade hub 26 and the regions adjacent thereto.
FIG. 4 is a cross-sectional view of the slot region 30 of a straddle-mount attachment structure. FIG. 5 is a cross-sectional view of the straddle-mount region 32 of a straddle-mount attachment architecture. The corresponding blade 28 for each attachment architecture region is omitted from FIGS. 4 and 5.
FIG. 5 also shows a number of ultrasound transducers 40A, 40B, 40C respectively placed at positions P1, P2, and P3. The transducers 40A, 40B, 40C are used to test the straddle-mount attachment structure for service induced flaws.
Ultrasonic testing procedures are commonly used to examine turbine components for the purpose of detecting and characterizing service induced flaws. The technique involves applying high frequency sound waves to a structure of interest. When the sound waves interact with an object that has a significant difference in acoustic impedance (the product of density and velocity) from that of the propagation medium, a portion of the sound is either reflected or diffracted back to the source from which the sound originated. Measurement and evaluation of the returned sound pattern permits determination of the presence and characteristics of the reflecting medium.
For ultrasonic techniques to work it is necessary to discriminate between object architecture and flaws in the object architecture. This discrimination is readily accomplished when the object architecture is known. In the case of straddle-mount rotor blade attachment structures, only the manufacturer knows the precise structure architecture. Thus, only a manufacturer is aware of the proper positions for the transducers used in ultrasonic testing, for example, positions P1, P2, and P3 in FIG. 5. Manufacturers are reluctant to disclose this information because it is generally considered a trade secret. In the absence of information regarding attachment architecture, it is difficult to discriminate between service induced flaws and attachment architecture. Thus, it would be highly desirable to provide a method and apparatus for physically characterizing a non-visible rotor blade attachment structure. This information could then be used to perform ultrasonic testing of the rotor blade attachment structure.